Endothermy, the Heat Within

By Tanya Brunner, MS

The media often refers to sharks as “cold-blooded.” While it is an inaccurate description of their personality, it is a proper description of their biology. Most sharks, like nearly all fish, are cold-blooded: in other words, the temperature of their blood is the same as the surrounding, or ambient, water temperature. This is very different from mammals, including humans, who maintain an internal temperature that is higher than their environment.

But all sharks are not created equal.

A small group of sharks called the Laminids have evolved the ability to maintain their body temperature a few degrees higher than their environment.

The Laminids include the infamous great white shark, mako, salmon, and porbeagle sharks. Pelagic tunas and billfish, like swordfish and marlin, are well known for this physical capability (Weng and Block 2004), and research has shown that even some species of thresher shark can do so as well (Bernal et al. 2005).

The ability to maintain a higher body temperature makes Lamnids not cold-blooded, but instead endothermic. The word “endothermy” means “heat within.” It refers to their ability to generate their own heat instead of relying on a heat source like their ectothermic relatives.

How do they do it?

When we slip under our bedcovers in the winter, our own body makes and keeps us warm. In other words, our active metabolism breaks down energy into heat, warming up the space underneath the covers. Lamnid sharks have the same ability to harness the heat their body produces through metabolism and re-distribute it to important muscles and to their eyes and brain (Dickson and Graham 2004).

In fish, heating up and cooling down happens at the gills. This is also the site of gas exchange, or breathing. When fish breathe, oxygen passes from the water into the blood that is within their gills. This blood leaves the gills and circulates throughout the fish’s body, supplying oxygen to all muscles and organs. The same process is used to warm and cool the body; the blood passing through the gills can be cooled down by colder water that rushes over the gills, or warmed by surrounding warm water.

Lamnid sharks employ a system called rete mirabile, a specific arrangement of blood vessels that allows warm blood coming from large swimming muscles to transfer heat to the cold blood that is coming from the gills and is on its way to deliver oxygen to the swimming muscles (Helfman et al. 1997).

This way, any blood entering the swimming muscles is always somewhat warm and certainly warmer than the water the shark is swimming in.

Lamnids also have retia, this special vessel arrangement, near their eyes. These retia are specifically intended to warm the eyes and brain (Helfman et al. 1997), which can improve the resolution of the images the eye picks up and help detect movement, like prey swimming by (Fritsches et al. 2005). For mako sharks, who travel vertically and come across very different temperatures in a short time, the retained warmth is especially important to keep the eyes and brain stabilized (Wolf et al. 1988).

Additionally, Lamnids also have a vein that transports warm blood from the swimming muscles to their spinal cord in order to keep their central nervous system warm and operating consistently well.

What’s the benefit?

The benefits of endothermy might seem obvious for us: for one, when we are cold in the winter, our body heat keeps us warm under the covers of our bed. How does it help great whites and their relatives?

These sharks can inhabit a wider range of environments with varying temperatures (Helfman et al. 1997); they are not restricted to living in the tropics, for example.

Warm bodies also keep biochemical reactions working smoothly (Helfman et al. 1997). For example, great whites have been reported to maintain a stomach temperature that is more than an astounding 14 degrees Celsius higher than the surrounding water temperature (Goldman 1997). Higher temperatures allow reactions to go faster and in the case of great whites, it increases their digestion rate.

When we exercise, our muscles become warmer. Warmer muscles, in turn, allow us to run faster and longer. The same goes for sharks; warm muscles allow them to swim faster and longer, called burst and sustained swimming. This also helps them when they are pursuing prey (Helfman et al. 1997), especially in colder water.

Are there costs to being endothermic?

Generating enough heat to keep your body warm means that these sharks have to hunt more in order to eat much more food than ectothermic sharks. Also, assigning so much of your food energy to keeping muscles warm can mean that less of it is used to grow and reproduce (Helfman et al. 1997).

Endothermy among sharks has allowed the Lamnids to evolve into highly active organisms capable of inhabiting a variety of environments. Even though being endothermic requires these sharks to eat a lot, it has several advantages: these sharks can see better, digest faster, and swim for long periods at high speeds.